1. Field of the Invention
This invention relates to polyvinyl alcohol microgels and, more particularly, to dry precursor blends which may be used to prepare polyvinyl alcohol microgels.
2. Description of the Prior Art
Certain materials such as starch, sodium carboxymethylcellulose, sodium alginate, and polyvinyl alcohol, with or without the addition of clay, have been used as surface sizes for paper and paperboard. By "surface size" is meant a material which is applied to the surface of the paper or paperboard, thereby forming a barrier which prevents or retards wicking and adsorption of ink and other fluids into the paper or paperboard. Surface sizes are to be distinguished from internal sizes which are added in the process of making the sheet.
Although polyvinyl alcohol is many times more expensive than starch on a per pound basis, it has been shown that polyvinyl alcohol can perform as well as, or in many cases better than, starch as a surface size for paper or paperboard on an equal cost basis. However, in most use areas the improvement in performance has not been sufficient to cause former starch users to switch to polyvinyl alcohol. At present polyvinyl alcohol is definitely preferred to starch only in specialty areas where quality is important.
There is evidence that polyvinyl alcohol solutions readily penetrate into paper and paperboard. It is felt that even better performance for polyvinyl alcohol would be demonstrated if its "hold out" characteristics could be improved; that is, if penetration of polyvinyl alcohol size solutions into paper and paperboard could be substantially reduced or eliminated.
It is known that polyvinyl alcohol can be gelled by the addition of various gelling agents such as Congo Red, Direct Orange 8 (Colour Index 22120), Direct Green 12 (Colour Index 30290), borax, boric acid, and various compounds of aluminum, chromium, copper, iron, titanium, vanadium and zirconium. In U.S. Pat. Nos. 2,720,468; 3,318,856; and 3,492,250 the use of various organo-titanium complexes to gel polyvinyl alcohol is taught. Since these gels are amorphous-to-rigid solids they are not useful as surface sizes for paper or as textile warp sizes.
In U.S. Pat. No. 3,318,856, Alden J. Deyrup discloses the use of a dry blend containing titanium oxalate to prepare solid gels of polyvinyl alcohol. It is stated that the dry blend must be dissolved in water at a pH of about 6. This acidic pH may be obtained by the addition of certain acids. It is further stated that acids which form complexes or insoluble precipitates with tetravalent titanium, such as citric acid, should not be used.
U.S. Pat. No. 3,318,856 also teaches a system wherein polyvinyl alcohol is present with two different polyvinyl alcohol gelling agents, one gelling agent reinforcing the other. In Example 3 of said patent boric acid functions as the initial gelling agent. Tetravalent titanium, present as potassium titanium oxalate, is the primary gelling agent used to form the rigid polyvinyl alcohol based foam described in Example 3 which has a Brookfield viscosity in excess of 2000 centipoises. The gelled skin formed in minutes on exposure to air also has a Brookfield viscosity in excess of 2000. Deyrup U.S. Pat. No. 3,492,250 in column 4, lines 48 to 65, explains the function of two polyvinyl alcohol gelling agents, including boric acid.
Deyrup U.S. Pat. No. 3,492,250, in Example 4, discloses in a solution containing polyvinyl alcohol the presence of a polysaccharide thickener (Abbott Laboratories B 1459) together with Congo Red as the gelling agent. The solution of Example 4 does not contain a tetravalent titanium compound and the polysaccharide is a xanthan gum as disclosed in Whistler and BeMiller, Industrial Gums, second edition, Academic Press, New York, VII, 1. History on page 486, 1973. A repeat of Example 4 shows that the xanthan gum polysaccharide does not crosslink with the Congo Red gelling agent present in the solution. The Brookfield viscosity at 60 rpm of the solution of Example 4 containing Congo Red was found to be 70 centipoises while the Brookfield viscosity at 60 rpm of a solution without Congo Red was found to be 73 centipoises.
In a copending application of Alden J. Deyrup, Ser. No. 259,552, filed June 5, 1972, polyvinyl alcohol microgels are claimed which exhibit improved hold out characteristics over aqueous polyvinyl alcohol solutions. These microgels are stable fluids comprising water and polyvinyl alcohol partially crosslinked with tetravalent titanium ions and having a Brookfield viscosity of about 1.15 to 2000 centipoises. The polyvinyl alcohol in these microgels is crosslinked sufficiently to increase the viscosity of the aqueous polyvinyl alcohol solution from which they are derived by at least about 15 percent, but insufficiently to increase the viscosity above about 2000 centipoises. This intermediate amount of crosslinking is referred to herein as partial crosslinking. It is believed that these microgels are entanglements of partially cross-linked polyvinyl alcohol chains possessing colloid-like properties dispersed in water.
In my copending application, Ser. No. 486,900, filed July 9, 1974, polyvinyl alcohol-polysaccharide microgels are claimed which provide sized paper having improved printing quality over the same paper sized with the polyvinyl alcohol microgels claimed in Deyrup U.S. Pat. application Ser. No. 259,552.
The microgels described in these two copending patent applications are prepared by partially crosslinking an aqueous solution of polyvinyl alcohol or a mixture of polyvinyl alcohol and one or more polysaccharide selected from the group consisting of starch and sodium carboxymethylcellulose using a titanium complex. Since the resulting microgels contain a large amount of water, it is not economical to transport them long distances. Moreover, they should be prepared only a short time before they are to be used, since they may not be stable for more than several days. Accordingly, it is preferred that these microgels be supplied on a commercial basis as a dry blend of microgel precursor ingredients. For example, polyvinyl alcohol can be dry mixed with a suitable solid titanium complex to form the commercial product. When ready for use, this blend is dissolved in water, whereupon the microgel is formed.